At Lok Nayak Hospital, on the edge of the chaotic and densely peopled nest of lanes that is Old Delhi, a human tide flows through the entrance gate every morning and crowds inside on the lobby floor. “Who could see this and not be worried about the population of India?” a surgeon named Chandan Bortamuly asked one afternoon as he made his way toward his vasectomy clinic. “Population is our biggest problem.” Removing the padlock from the clinic door, Bortamuly stepped into a small operating room. Inside, two men lay stretched out on examination tables, their testicles poking up through holes in the green sheets. A ceiling fan pushed cool air from two window units around the room.

Bortamuly is on the front lines of a battle that has been going on in India for nearly 60 years. In 1952, just five years after it gained independence from Britain, India became the first country to establish a policy for population control. Since then the government has repeatedly set ambitious goals—and repeatedly missed them by a mile. A national policy adopted in 2000 called for the country to reach the replacement fertility of 2.1 by 2010. That won’t happen for at least another decade. In the UN’s medium projection, India’s population will rise to just over 1.6 billion people by 2050. “What’s inevitable is that India is going to exceed the population of China by 2030,” says A. R. Nanda, former head of the Population Foundation of India, an advocacy group. “Nothing less than a huge catastrophe, nuclear or otherwise, can change that.”

Sterilization is the dominant form of birth control in India today, and the vast majority of the procedures are performed on women. The government is trying to change that; a no-scalpel vasectomy costs far less and is easier on a man than a tubal ligation is on a woman. In the operating theater Bortamuly worked quickly. “They say the needle pricks like an ant bite,” he explained, when the first patient flinched at the local anesthetic. “After that it’s basically painless, bloodless surgery.” Using the pointed tip of a forceps, Bortamuly made a tiny hole in the skin of the scrotum and pulled out an oxbow of white, stringy vas deferens—the sperm conduit from the patient’s right testicle. He tied off both ends of the oxbow with fine black thread, snipped them, and pushed them back under the skin. In less than seven minutes—a nurse timed him—the patient was walking out without so much as a Band-Aid. The government will pay him an incentive fee of 1,100 rupees (around $25), a week’s wages for a laborer.

The Indian government tried once before to push vasectomies, in the 1970s, when anxiety about the population bomb was at its height. Prime Minister Indira Gandhi and her son Sanjay used state-of-emergency powers to force a dramatic increase in sterilizations. From 1976 to 1977 the number of operations tripled, to more than eight million. Over six million of those were vasectomies. Family planning workers were pressured to meet quotas; in a few states, sterilization became a condition for receiving new housing or other government benefits. In some cases the police simply rounded up poor people and hauled them to sterilization camps.

The excesses gave the whole concept of family planning a bad name. “Successive governments refused to touch the subject,” says Shailaja Chandra, former head of the National Population Stabilisation Fund (NPSF). Yet fertility in India has dropped anyway, though not as fast as in China, where it was nose-diving even before the draconian one-child policy took effect. The national average in India is now 2.6 children per woman, less than half what it was when Ehrlich visited. The southern half of the country and a few states in the northern half are already at replacement fertility or below.

In Kerala, on the southwest coast, investments in health and education helped fertility fall to 1.7. The key, demographers there say, is the female literacy rate: At around 90 percent, it’s easily the highest in India. Girls who go to school start having children later than ones who don’t. They are more open to contraception and more likely to understand their options.

SO FAR THIS APPROACH, held up as a model internationally, has not caught on in the poor states of northern India—in the “Hindi belt” that stretches across the country just south of Delhi. Nearly half of India’s population growth is occurring in Rajasthan, Madhya Pradesh, Bihar, and Uttar Pradesh, where fertility rates still hover between three and four children per woman. More than half the women in the Hindi belt are illiterate, and many marry well before reaching the legal age of 18. They gain social status by bearing children—and usually don’t stop until they have at least one son.

As an alternative to the Kerala model, some point to the southern state of Andhra Pradesh, where sterilization “camps”—temporary operating rooms often set up in schools—were introduced during the ’70s and where sterilization rates have remained high as improved hospitals have replaced the camps. In a single decade beginning in the early 1990s, the fertility rate fell from around three to less than two. Unlike in Kerala, half of all women in Andhra Pradesh remain illiterate.

Amarjit Singh, the current executive director of the NPSF, calculates that if the four biggest states of the Hindi belt had followed the Andhra Pradesh model, they would have avoided 40 million births—and considerable suffering. “Because 40 million were born, 2.5 million children died,” Singh says. He thinks if all India were to adopt high-quality programs to encourage sterilizations, in hospitals rather than camps, it could have 1.4 billion people in 2050 instead of 1.6 billion.

Critics of the Andhra Pradesh model, such as the Population Foundation’s Nanda, say Indians need better health care, particularly in rural areas. They are against numerical targets that pressure government workers to sterilize people or cash incentives that distort a couple’s choice of family size. “It’s a private decision,” Nanda says.

In Indian cities today, many couples are making the same choice as their counterparts in Europe or America. Sonalde Desai, a senior fellow at New Delhi’s National Council of Applied Economic Research, introduced me to five working women in Delhi who were spending most of their salaries on private-school fees and after-school tutors; each had one or two children and was not planning to have more. In a nationwide survey of 41,554 households, Desai’s team identified a small but growing vanguard of urban one-child families. “We were totally blown away at the emphasis parents were placing on their children,” she says. “It suddenly makes you understand—that is why fertility is going down.” Indian children on average are much better educated than their parents.

That’s less true in the countryside. With Desai’s team I went to Palanpur, a village in Uttar Pradesh—a Hindi-belt state with as many people as Brazil. Walking into the village we passed a cell phone tower but also rivulets of raw sewage running along the lanes of small brick houses. Under a mango tree, the keeper of the grove said he saw no reason to educate his three daughters. Under a neem tree in the center of the village, I asked a dozen farmers what would improve their lives most. “If we could get a little money, that would be wonderful,” one joked.

The goal in India should not be reducing fertility or population, Almas Ali of the Population Foundation told me when I spoke to him a few days later. “The goal should be to make the villages livable,” he said. “Whenever we talk of population in India, even today, what comes to our mind is the increasing numbers. And the numbers are looked at with fright. This phobia has penetrated the mind-set so much that all the focus is on reducing the number. The focus on people has been pushed to the background.”

It was a four-hour drive back to Delhi from Palanpur, through the gathering night of a Sunday. We sat in traffic in one market town after another, each one hopping with activity that sometimes engulfed the car. As we came down a viaduct into Moradabad, I saw a man pushing a cart up the steep hill, piled with a load so large it blocked his view. I thought of Ehrlich’s epiphany on his cab ride all those decades ago. People, people, people, people—yes. But also an overwhelming sense of energy, of striving, of aspiration.

THE ANNUAL meeting of the Population Association of America (PAA) is one of the premier gatherings of the world’s demographers. Last April the global population explosion was not on the agenda. “The problem has become a bit passé,” Hervé Le Bras says. Demographers are generally confident that by the second half of this century we will be ending one unique era in history—the population explosion—and entering another, in which population will level out or even fall.

But will there be too many of us? At the PAA meeting, in the Dallas Hyatt Regency, I learned that the current population of the planet could fit into the state of Texas, if Texas were settled as densely as New York City. The comparison made me start thinking like Leeuwenhoek. If in 2045 there are nine billion people living on the six habitable continents, the world population density will be a little more than half that of France today. France is not usually considered a hellish place. Will the world be hellish then?

Some parts of it may well be; some parts of it are hellish today. There are now 21 cities with populations larger than ten million, and by 2050 there will be many more. Delhi adds hundreds of thousands of migrants each year, and those people arrive to find that “no plans have been made for water, sewage, or habitation,” says Shailaja Chandra. Dhaka in Bangladesh and Kinshasa in the Democratic Republic of the Congo are 40 times larger today than they were in 1950. Their slums are filled with desperately poor people who have fled worse poverty in the countryside.

Whole countries today face population pressures that seem as insurmountable to us as India’s did to Ehrlich in 1966. Bangladesh is among the most densely populated countries in the world and one of the most immediately threatened by climate change; rising seas could displace tens of millions of Bangladeshis. Rwanda is an equally alarming case. In his book Collapse, Jared Diamond argued that the genocidal massacre of some 800,000 Rwandans in 1994 was the result of several factors, not only ethnic hatred but also overpopulation—too many farmers dividing the same amount of land into increasingly small pieces that became inadequate to support a farmer’s family. “Malthus’s worst-case scenario may sometimes be realized,” Diamond concluded.

Many people are justifiably worried that Malthus will finally be proved right on a global scale—that the planet won’t be able to feed nine billion people. Lester Brown, founder of Worldwatch Institute and now head of the Earth Policy Institute in Washington, believes food shortages could cause a collapse of global civilization. Human beings are living off natural capital, Brown argues, eroding soil and depleting groundwater faster than they can be replenished. All of that will soon be cramping food production. Brown’s Plan B to save civilization would put the whole world on a wartime footing, like the U.S. after Pearl Harbor, to stabilize climate and repair the ecological damage. “Filling the family planning gap may be the most urgent item on the global agenda,” he writes, so if we don’t hold the world’s population to eight billion by reducing fertility, the death rate may increase instead.

Eight billion corresponds to the UN’s lowest projection for 2050. In that optimistic scenario, Bangladesh has a fertility rate of 1.35 in 2050, but it still has 25 million more people than it does today. Rwanda’s fertility rate also falls below the replacement level, but its population still rises to well over twice what it was before the genocide. If that’s the optimistic scenario, one might argue, the future is indeed bleak.

But one can also draw a different conclusion—that fixating on population numbers is not the best way to confront the future. People packed into slums need help, but the problem that needs solving is poverty and lack of infrastructure, not overpopulation. Giving every woman access to family planning services is a good idea—“the one strategy that can make the biggest difference to women’s lives,” Chandra calls it. But the most aggressive population control program imaginable will not save Bangladesh from sea level rise, Rwanda from another genocide, or all of us from our enormous environmental problems.

Global warming is a good example. Carbon emissions from fossil fuels are growing fastest in China, thanks to its prolonged economic boom, but fertility there is already below replacement; not much more can be done to control population. Where population is growing fastest, in sub-Saharan Africa, emissions per person are only a few percent of what they are in the U.S.—so population control would have little effect on climate. Brian O’Neill of the National Center for Atmospheric Research has calculated that if the population were to reach 7.4 billion in 2050 instead of 8.9 billion, it would reduce emissions by 15 percent. “Those who say the whole problem is population are wrong,” Joel Cohen says. “It’s not even the dominant factor.” To stop global warming we’ll have to switch from fossil fuels to alternative energy—regardless of how big the population gets.

The number of people does matter, of course. But how people consume resources matters a lot more. Some of us leave much bigger footprints than others. The central challenge for the future of people and the planet is how to raise more of us out of poverty—the slum dwellers in Delhi, the subsistence farmers in Rwanda—while reducing the impact each of us has on the planet.

The World Bank has predicted that by 2030 more than a billion people in the developing world will belong to the “global middle class,” up from just 400 million in 2005. That’s a good thing. But it will be a hard thing for the planet if those people are eating meat and driving gasoline-powered cars at the same rate as Americans now do. It’s too late to keep the new middle class of 2030 from being born; it’s not too late to change how they and the rest of us will produce and consume food and energy. “Eating less meat seems more reasonable to me than saying, ‘Have fewer children!’ ” Le Bras says.

How many people can the Earth support? Cohen spent years reviewing all the research, from Leeuwenhoek on. “I wrote the book thinking I would answer the question,” he says. “I found out it’s unanswerable in the present state of knowledge.” What he found instead was an enormous range of “political numbers, intended to persuade people” one way or the other.

For centuries population pessimists have hurled apocalyptic warnings at the congenital optimists, who believe in their bones that humanity will find ways to cope and even improve its lot. History, on the whole, has so far favored the optimists, but history is no certain guide to the future. Neither is science. It cannot predict the outcome of People v. Planet, because all the facts of the case—how many of us there will be and how we will live—depend on choices we have yet to make and ideas we have yet to have. We may, for example, says Cohen, “see to it that all children are nourished well enough to learn in school and are educated well enough to solve the problems they will face as adults.” That would change the future significantly.

The debate was present at the creation of population alarmism, in the person of Rev. Thomas Malthus himself. Toward the end of the book in which he formulated the iron law by which unchecked population growth leads to famine, he declared that law a good thing: It gets us off our duffs. It leads us to conquer the world. Man, Malthus wrote, and he must have meant woman too, is “inert, sluggish, and averse from labour, unless compelled by necessity.” But necessity, he added, gives hope:

“The exertions that men find it necessary to make, in order to support themselves or families, frequently awaken faculties that might otherwise have lain for ever dormant, and it has been commonly remarked that new and extraordinary situations generally create minds adequate to grapple with the difficulties in which they are involved.”

Seven billion of us soon, nine billion in 2045. Let’s hope that Malthus was right about our ingenuity.

Robert Kunzig is National Geographic’s senior editor for the environment.